Method of and apparatus for the electrolytic extraction of gold from a gold-bearing solution



Nov. 13, 1962 A. LEIBOWlTZ 3,063,921

METHOD OF AND APPARATUS FOR THE ELECTROLYTIC EXTRACTION OF GOLD FROM AGOLD-BEARING SOLUTION Unite States 3,063,921 METHQD OF AND APPARATUS FORTHE ELEC- TROLYTIC EXTRACTION OF GOLD FRGM A GOLD-BEARING SOLUTION AlecLeihowitz, Johannesburg, Transvaal, Republic of South Africa, assignorto Rand Mines Limited, Johannesburg, Transvaal, Republic of SouthAfrica, a limited-liability company of the Republic of South Africa,

and Alec Leibowitz, Hugh McLelian Husted, Lionel Siderslry, and KennethNeil Kitsch, a partnership, Jo-

hannesburg, Transvaal, Republic of South Africa Filed Apr. 20, 196i),Ser. No. 23,427 Claims priority, application Republic of South AfricaAug. 6, 1957 6 Claims. (Cl. 2ll4113) This invention relates to a methodof electrolytic extraction of gold from a gold-bearing solution, andthis application is a continuation-in-part of patent application SerialNumber 752,233, filed July 31, 1958, now abandoned. The term goldbearing solution means an aqueous solution of a gold solvent, such ascyanide, which has been in contact with gold and has caused itsdissolution. The said solution may either be clarified or it may containcomminuted ore from which the gold has been wholly or partiallydissolved, or is in process of being dissolved, but in practice it willinvariably contain surplus gold solvent.

In the past, attempts to extract gold electrolytically from agold-bearing solution which had been obtained after cyanide treatment ofthe comminuted gold-bearing ore, have been unsatisfatcory in thepresence of such ore. It has been assumed by those skilled in the artthat failure has been due to:

(a) The difliculty experienced in obtaining an adherent deposit of goldon the cathode.

(b)The abrasion of the deposited gold by the ore passing over astationary cathode.

(c) Cathode corrosion.

(d) Inability to treat large quantities.

The electrolytic extraction of gold from a clarified solution has beenunsatisfactory hitherto for the last two reasons enumerated above.

It is known that, if a clarified gold-bearing solution is passed rapidlythrough stationary cathodes made of iron gauze, the actual extraction ofthe gold by an electrolytic process becomes more satisfactory. Owing tothe rapid passage of the solution through the cathode, the currentdensity can be increased to an extent which will increase the rate ofdeposition without producing a powdery deposit of gold on the cathodes.It is believed that the efiiciency of the extraction is such that only asmall residue of gold remains unextracted. However, in order to obtainsatisfactory results by this method, a large number of cathodes isrequired and the solution has to pass through them very rapidly indeed.The result is that the size and complexity of the plant required opposethe practical application of the method.

It is an object of the present invention to provide a satisfactoryelectrolytic method of extraction which minimizes the abovedisadvantages.

It is also an object of the invention to provide a method for theelectrolytic extraction of gold by means of which substantially completeextraction may be effected.

It is another object of the invention to provide a method for theelectrolytic extraction of gold by means of which substantially completeextraction may be effected in the'prese'nce of comminuted ore containedin the goldbearing solution.

According to one aspect of the invention, there is provided a method ofelectrolytically extracting gold from a gold-bearing solution whichcomprises the step of moving a liquid-pervious cathode, the entiredeposition zone fihfifill Patented Nov. 13, 1952 of which is alwaysimmersed in the solution, continuously through the solution while adirect electric current flows between an anode and the cathode, at aspeed which is high enough to deposit an adherent metallic layer and lowenough to prevent substantial resolution and dislodgrnent of thedeposited gold.

According also to the invention, the method above defined is applied tothe electrolytic extraction of gold from a gold bearing solution in thepresence of commi nuted ore.

If the electrolytic extraction is carried out in a vessel in which goldbearing comminuted ore from which the gold is in process of beingdissolved is present, and if substantially complete dissolution isrequired, it has been found essential to aerate the solution and toagitate it by independent mechanical means in the event that aerationand/or cathode movement do not themselves perform the function ofagitation. At the same time, it is equally essential to prevent airbubbles from flowing over the surface of the moving cathode or frombeing entrapped thereby. This may be achieved, for example, by bafilesso arranged that they do not prevent the pulp from moving freely withthe cathode.

It is theoretically possible to achieve satisfactory results byimparting a reciprocating movement to the cathode, provided the leadingedges of the cathode structure, which exist by reason of the fact thatthecathode is liquid pervious, are caused to face the direction ofmovement. It is preferred, however, to rotate the cathode in thesolution.

According also to the invention, the initial speed of movement of thecathode is reduced during the extraction process.

According to another aspect of the invention, there is providedapparatus for carrying out the method abovedefined comprising a tank tocontain the solution and a cathode and an anode, which anode may, ifdesired, be provided by the tank itself. Said anode must besubstantially parallel to the cathode and substantially at least as longas and substantially co-extensive with the longitudinal extent of thedeposition zone of the cathode. Said cathode is liquid-pervious, havingits entire deposition zone always immersed in the solution duringoperation, and is so constructed that any point on its mean peripheralsurface moves at substantially the same speed. Also included are meansfor moving the cathode and means for causing a direct current to flowbetween the anode and the cathode.

If the electrolytic extraction is taking place in a tank which alsocontains gold bearing comminuted ore from which the gold is in processof being dissolved, and if substantially complete dissolution isrequired, the tank must be provided with an inlet through which airunder pressure may be introduced to aerate and agitate the solution andalso with battle means to prevent air bubbles from flowing over thesurface of the cathode and from being entrapped thereby, said bafilemeans being arranged so as not to prevent the pulp from moving freelywith the cathode.

The cathode may conveniently comprise a gauze cylinder. The longitudinalelements of the moving gauze structure will, as will be appreciated,have both leading and trailing edges. Such cylinder may, for example, besupported on a spider and caused to rotate about a vertical axis.Alternatively, it may be supported on rollers and driven by meanslocated externally of the tank. The gauze may be made of anyelectrically conductive metal of adequate structural strength such as,for example, stainless steel or copper. The advantage of constructingthe cathode of stainless steel is that this material is inert and, inconsequence, corrosion which might occur as a result of intermittent useof the apparatus or during the deplating stage referred to below isavoided.

It has been found that iron anodes are corroded as a result of exposureto air and/ or water, and/ or as a result of the electrolytic action,and/or as a result of impurities introduced by the comminuted ore. Ifthe tank is made of a conducting material which is not unduly affectedby such corrosion, then the tank itself may serve as the anode.Otherwise, an independent anode may be used. Such independent anode maycomprise open-ended cylinders suspended in the tank, without being inelectrical contact therewith, to a minimum depth such as will enablethem to surround the deposition zone of the rotating cathode. The anodesmay be constructed from any suitable resistant conducting material suchas, for example, hard carbon, graphite, or peroxidized lead.Unperoxidized lead will, of course, become peroxidized in the tank dueto the anodic electrolytic effect.

The invention and the manner in which the same is to be performed willnow be further described, purely by way of example, with reference tothe accompanying drawing in which:

FIG. 1 is a diagrammatic cross-sectional elevation of apparatus suitablefor the electrolytic extraction of gold in accordance with theinvention.

FIG. 2 is a cross-section on line IIIl of FIG. 1.

Referring to the drawing, 1 is a tank to contain goldbearing pulp. 2 isa stainless steel gauze cathode in the form of a cylinder supported by aspider 3 and rotatable on a vertical axis by means of a variable speedprime mover 4 through a reduction gear 5. 6 is a stationary lead anodein the form of an open-ended cylinder disposed around the cathode 2.

The anode 6 is parallel to the cathode 2 and as long as and co-extensivewith the longitudinal extent of the cathode 2. 7 is an inlet forcompressed air and 8 a baffie which prevents air bubbles from flowingover the surface of the cathode or from being entrapped thereby. Thisprevents resolution of the gold due to interference with the uniformityof the current flowing between the anode and cathode. 9 is a source ofDC. current and 10 an outlet for draining the tank.

In tests conducted, the tank was 4 feet high and 15 inches in diameter.The cathode was inches diameter and 6 inches high, its effective surfacearea being 70 square inches. The voltage applied was 12 volts and thecurrent 1.4 amperes. The initial gold content was 1 dwt. per liquid tonand after 24 hours the gold residue was 0.04 dwt. per liquid ton. Thedensity of the pulp was 1.6.

During removal of the gold from the solution from 1 dwt. down to 0.2dwt., the speed of rotation of the cathode was 200 r.p.rn. and from 0.2dwt. down to 0.04 dwt., the speed was reduced to 120 rpm.

It has been found that if a dark, friable deposit of gold on the cathodeis obtained, which type of deposit is not required, it is an indicationthat the speed of rotation of the cathode is too low and should beincreased in order to obtain an adherent deposited layer. Also, a lowspeed will mean that extraction proceeds but slowly i.e. that theefiiciency of extraction is low. It might therefore be supposed that,compatible with the current density employed, the higher the speed ofrotation of the cathode the better. It has been found, however, thatthis is not the case. Two factors impose a practical upper limit on thespeed of rotation. These are resolution of the deposited gold and, inthe case where extraction takes place in the presence of comminuted ore,abrasion.

As regards resolution, it is known that when gold particles aredissolved conventionally in a very dilute cyanide solution, the speed ofmovement of the gold particles is a determining factor in the rate ofsolution. H. A. White (Journal of the Chemical, Metallurgical and MiningSociety of South Africa, vol. 35, pp. 1-11) reaches the 4 conclusionthat gold particles of 260 mesh size will dissolve at the rate of 8.8 i

milligrams per square centimetre of surface per 24 hours, V being theaverage velocity of particle settlement in centimetres per second, and lthe average length of a particle in microns.

Tests with a solution containing 1 dwt./ton of gold showed that theeffective degree of extraction was initially much more satisfactory atreasonably high cathode speeds. Thus, with a peripheral speed of 30 feetper minute, the extraction rate was 0.045 dwts. per ton per hour,Whereas with a speed of 300 feet per minute it was 0.20 dwts. per tonper hour, an increase in the extraction rate of 340%.

However, when the gold concentration becomes low, the reverse is foundto occur. With a solution containing 0.2 dwts. per ton and a peripheralspeed of 30 feet per minute, the extraction rate was 0.013 dwts. per tonper hour. Raising the speed to 300 feet per minute lowered theextraction rate to 0.008 dwts. per ton per hour, a decrease of about38%.

Visual examination indicated that at high speeds, and when the goldconcentration had reached a low level, resolution of the gold which hadbeen deposited at the higher concentrations on the trailing edges of thevertical members of the cathode gauze took place.

If, therefore, the deposit on the trailing edges of the cathodestructure begins to be removed owing to resolution, especially at lowconcentrations, it is an indication that the speed of rotation is toohigh. Hence, in the tests, the reduction in speed when the concentrationfell to 0.2 dwts. per liquid ton.

As regards abrasion during extraction from a solution containingcomminuted ore, it is in practice impossible to grind or mill ore fineenough entirely to eliminate abrasion of the deposited gold at highcathode speeds. It is therefore essential that the electrolyte shouldcontain the gold solvent so that any abraded gold can redissolve priorto redeposition. Furthermore, although it has been established that ahigh cathode speed is effective in improving the rate of extraction,this approach must be moderated in view of the amount of abrasion whichthis high speed can cause.

In practice, therefore, it is necessary in order to obtain substantiallycomplete extraction, to strike a balance between too high a speed whichwill cause material resolution, and also abrasion if extraction istaking place in the presence of comminuted ore, and too low a speedwhich will induce a loose, powdery deposit and poor extractionefficiency.

It has been shown that while a relatively high speed may be employed forrapid initial extraction, that speed must be reduced in order to inhibitresolution at low concentrations. The reduction in speed will bring withit the benefit of a reduced tendency to abrasion and the final speed toobtain substantially complete extraction must be associated with theelimination of abrasion.

Cathode movement is essential to obtain good admixture of the pulp orelectrolyte and to establish maximum contact between the cathode surfaceand the gold ions in solution. It is therefore necessary to obtainmaximum movement within the scope of abrasion and resolutionlimitations. To effect this, constructional design must aim atminimizing abrasion. The cathode movement will cause the pulp to flowwith it, thereby reducing impact between the ore particles and thecathode surface. To maintain this stream of pulp flowing with the movingcathode, it is essential that no obstacle exists in the tank which willdivert this flow from or against the direction of cathode movement.

As regards resolution from the trailing edges of the cathode structure,it has also been found that it can be minimized by increasing thevoltage, reducing the thickness of the cathode and/or providing anodesboth inside and outside the cathode. If anodes are provided inside thecathode, they may be caused to rotate therewith.

In the test described with reference to the drawing, perfect adherenceof the gold to the cathode was obtained and it was convincinglydemonstrated that there was no tendency for the gold to be removed byabrasion.

After extraction has proceeded to the desired extent, the deposited goldmay be transferred from the cathode to concentrating members. This maybe done by gradually removing the cathode from the tank while thecurrent supply is still connected and Washing the cathode down with awater spray to prevent redissolution of the gold in the solutionadhering to it. Thereafter, the cathode is transterred to an aqueouscyanide solution having concentrating members of electrically conductivematerial located in it. The electrolytic process is then reversed bychanging the polarity of the current flowing through the cathode so thatthe gold is deplated from the cathode and deposited on the concentratingmembers. If desired, the cathode may be deplated after the treatment ofeach batch of gold-bearing solution, but it is possible for successivedepositions to take place on the cathode before it is deplated.

I claim:

1. A method of electrolytically extracting gold from a gold-bearingsolution which comprises the steps of moving a liquid-pervious cathode,which is always immersed in the solution, continuously through thesolution while causing a direct current to flow between an anode and thecathode, at a speed of movement of the cathode which is high enough todeposit an adherent metallic layer on the cathode and low enough tosubstantially prevent resolution and dislodgment of the deposited gold;and

adjusting the speed of movement of the cathode to a lower level duringthe extraction process to compensate for depletion of gold from saidsolution.

2. A method of electrolytically extracting gold from a gold-bearingsolution in the presence of comrninuted are which comprises the steps ofmoving a liquid pervious cathode, which is always immersed in thesolution, continuously through the solution while causing a directcurrent to flow between an anode and the cathode, at a speed of movementof the cathode which is high enough to deposit an adherent metalliclayer on the cathode and low enough to substantially prevent resolutionand dislodgment of the deposited gold; and then adjusting the speed ofmovement of the cathode to a lower level during the extraction processto compensate for depletion of gold from said solution.

3. The method of claim 1, in which the cathode is caused to rotate inthe solution.

4. The method of claim 1, in which the solution is an aqueous cyanidesolution.

5. The method of claim 2, in which the cathode is caused to rotate inthe solution.

6. The method of claim 2, in which the solution is an aqueous cyanidesolution.

References Qited in the tile of this patent UNITED STATES PATENTS640,718 Tatro et a1 Jan. 2, 1900 1,015,546 Davis Jan. 23, 1912 1,251,302Tainton Dec. 25, 1917 1,344,681 Dalziel June 29, 1920 OTHER REFERENCESElectro-Analysis by Edgar F. Smith, 4th edition 1907 (page 51

1. A METHOD OF ELECTROLYTICALLY EXTRACTING GOLD FROM A GOLD-BEARINGSOLUTION WHICH COMPRISES THE STEPS OF MOVING A LIQUID-PREVIOUS CATHODE,WHICH IS ALWAYS IMMERSED IN THE SOLUTION, CONTINUOUSLY THROUGH THESOLUTION WHILE CAUDING A DIRECT CURRENT TO FLOW BETWEEN AN ANODE AND THECATHODE, AT A SPEED OF MOVEMENT OF THE CATHODE WHICH IS HIGH ENOUGH TODEPOSIT AN ADHERENT METALLIC LAYER ON THE CATHODE AND LOW ENOUGH TOSUBSTANTIALLY PREVENT RESOLUTION AND DISLODGMENT OF THE DEPOSITED GOLD;AND ADJUSTING THE SPEED OF MOVEMENT OF THE CATHODE TO A LOWER LEVELDURING THE EXTRACTION PROCESS TO COMPENSATE FOR DEPLETION OF GOLD FROMSAID SOLUTION.